Manufacture and assembly of structures

ABSTRACT

The invention relates to a method of assembling structures and a component for use in such assemblies. The component is an electrically insulant, compressible impermeable cloth. It is formed by impregnating an electrically insulant woven fiber with a sealant, and curing to form the cloth. Structures, such as airframe structures, may be assembled by positioning the cloth between a substructure and an outer skin on at least part of a sub-structure, and then assembling the outer skin to the sub-structure with the cloth located between the substructure and the outer skin. This provides a greater impermeability and insulation across the structural joint. During assemble the cloth may also be compressed, allowing skin panels to be fixed to the substructure within close tolerances, and allowing steps between adjacent panels to be substantially reduced.

This application is the US national phase of international applicationPCT/GB02/04778 filed in English on 22 Oct. 2002, which designated theUS. PCT/GB02/04778 claims priority to GB Application No. 0126957.0 filed9 Nov. 2001. The entire contents of these applications are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the manufacture and assembly ofstructures, particularly those structures having an outer layer or skinsecured to or supported by a sub-structure and where the outer layer isrequired to conform within close tolerances to a predetermined profile.

2. Discussion of Prior Art

It is often desirable to assemble such structures by first providing thesub-structure, or skeletal framework, and then attaching panels to thesub-structure to form the outer layer or skin.

This type of structure is common in the design and manufacture ofaircraft where light-weight, high strength structures are required. Inthis industry it is additionally necessary to ensure that all the partsof the structure are manufactured within tight tolerances andfurthermore that the parts fit together so that the assembled structuremeets stringent accuracy requirements.

Any out of tolerance parts or inaccurately fitted parts will cause theadjacent parts being assembled to be fitted out of their required placein the structure, rendering the structure unacceptable. It is alsoimportant that adjacent parts of the outer skin do not have a stepbetween them so that the constituent panels and skins of the outer layerof the aircraft structure are flush with one another. Failing to providea substantially smooth aircraft outer layer can result in unwantedaerodynamic effects such as increased drag or turbulence.

To meet the strict tolerances required in aircraft construction forexample the underlying sub-structure may be made from machinablealuminium or titanium. The sub-structure may then be machined asnecessary to allow outer skins or panels to be fitted to it withoutadjacent skins or panels having a step between them. This method is notdesirable as any machining errors may cause the whole sub-structure tofail a quality assurance inspection and be rejected with consequent costand time penalties. Additionally underlying sub-structures areincreasingly being made from lightweight composite materials such ascarbon fibre reinforced plastics (CFRP) and these materials are notreadily machinable.

A method of producing structures to high accuracy requirements is known,and can be used with sub-structures made of either metal or CFRP. Inthis method, the surfaces of sub-structure to which panels are to beattached are coated with a filled, two component liquid adhesivematerial, with aluminium added to it. The liquid adhesive is cured onthe sub-structure, and is then machined to a desired thickness beforethe panels or skins are fixed to the sub-structure. The cured adhesivemay be machined to different thicknesses at different locations on thesub-structure so that, when the panels or skins are fixed to it there issubstantially no step between adjacent panels or skins.

Whilst this method produces structures having profiles with acceptableaccuracy, it has several disadvantages. Adhesive of this type is aviscous liquid which must be applied carefully to the sub-structure byhand using a spatula, so that it is distributed reasonably evenly withthe desired thickness and without creating air bubbles in the adhesive.Too much adhesive will result in a longer wait for curing and more timespent in machining than necessary. Adhesive of this type is difficult toapply in desired quantities because of its viscosity and furthermore,there are health and safety implications associated with its use.Personnel must be trained to use such adhesive and must be careful whenapplying it to the sub-structure. Also special tooling must bemanufactured, tailor made for each area to be panelled, to prevent theliquid adhesive from spreading to areas where it is not required, and togive guidance as to the thickness of the adhesive being applied. Becauseof the nature of this type of adhesive, the tooling must be coated witha release agent before use and cleaned thoroughly after use. Repeatedexposure to this coating and cleaning process causes the tooling todeteriorate rapidly after a relatively low number of uses, resulting intime lost and expense in manufacturing and fitting replacement tooling.

Furthermore, it is usually desirable to ensure that a joint between asubstructure and an outer skin is both insulated and sealed, to preventelectrical current and liquids respectively from flowing through thejoint. In the event of a lightning strike, current flowing from theouter skin through the joint to the substructure can damage theintegrity of the joint, and thereby compromise the safety of theaircraft. Liquid flowing through the joint can cause corrosion, whichmay weaken the joint and/or the adjacent substructure and/or the outerskin.

Known methods of producing structures involve bonding aresin-impregnated glass fibre cloth to the inner surface of the outerskin, to provide an insulative layer between the outer skin and thesubstructure. The substructure, or a layer formed thereon as describedabove, is then machined to the required tolerance, and finally sealantis applied between the substructure and the glass fibre cloth before thesubstructure and the outer skin are bonded or bolted together.

This method has several drawbacks. Firstly, the glass fibre cloth,having different material characteristics to the outer skin, can expandat a different rate to the outer skin, which may be carbon fibre forexample. Such differential expansion may cause delamination ordistortion of the outer skin, both of which have serious consequencesfor aircraft. Secondly, the sealant, which is generally obtainable as aviscous liquid, is often squashed out of the joint during assembly to anextent that the joint is rendered permeable. Thirdly, the machining ofthe substructure or the layer formed thereon to obtain the requiredtolerances is time consuming and has the difficulties and disadvantagesdescribed in detail above.

SUMMARY OF THE INVENTION

The present invention seeks to provide a method of and components forassembling structures having an outer layer supported by asub-structure, and where the outer layer is required to conform, withinclose tolerances, to a predetermined profile, with improved insulationand impermeability across the joint, and without the need to machine thesub-structure or a layer formed thereon to close tolerances.

According to the present invention in one aspect thereof, there isprovided a method of assembling a structure comprising at least thesteps of

-   -   providing a sub-structure,    -   providing an electrically insulant cloth which is impregnated        with a sealant that substantially prevents the passage of liquid        through the cloth, the impregnated electrically insulant cloth        being compressible,    -   assembling an outer layer with the sub-structure such that the        electrically insulant cloth lies substantially between the outer        layer and the sub-structure,    -   exerting pressure on the structure during assembly to cause the        impregnated electrically insulant cloth to compress to a desired        thickness, such that an outer surface of the structure is within        a required tolerance.

Preferably the electrically insulant cloth undergoes curing duringassembly of the structure. If required, the substructure, or a layerdisposed thereon, may be machined prior to assembly of the structure.

According to the invention in another aspect thereof, there is providedan electrically insulant cloth impregnated with a sealant thatsubstantially prevents the passage of liquid through the cloth, theimpregnated electrically insulant cloth being compressible. Preferablythe cloth is a glass fibre cloth. Advantageously the cloth iscompressible.

The cloth may comprise at least one layer of cloth impregnated with asealant and further comprise a layer comprising partially cured sealant.In this case, the layer comprising partially cured sealant may or maynot have substantially constant thickness. It may be advantageous tovary the thickness, particularly where the substructure is stepped, sothat the outer panel may still be fixed within the tolerance required.The cloth may further comprise a layer comprising precured sealant.Preferably the cloth has at least one surface having adhesiveproperties, enabling it to be affixed to the substructure withoutslipping during assembly of the structure.

According to the invention in another aspect thereof, there is provideda method of making an electrically insulant and impermeable clothcomprising at least the steps of substantially impregnating anelectrically insulant cloth with a sealant that substantially preventsthe passage of liquid through the cloth, curing the impregnatedelectrically insulant cloth, then adding a layer of sealant to the curedimpregnated electrically insulant cloth, and partially curing said layerof sealant. This provides a readily compressible layer having adhesiveproperties, enabling the cloth to be easily affixed to the substructure.

The method may alternatively comprise the further steps of adding afirst layer of sealant to a first side of the cured impregnatedelectrically insulant cloth, and fully curing the first layer ofsealant, then adding a second layer of sealant to a second side of saidcloth and partially curing the second layer of sealant. This provides acured impermeable layer on one side of the cloth and a readilycompressible, adhesive layer on the other side of the cloth.

According to the invention in another aspect thereof, there is provideda structural joint comprising a substructure and an outer layer, havingan intermediate layer disposed between the substructure and the outerlayer, wherein the intermediate layer is substantially impermeable andsubstantially compressible and substantially electrically insulant, theintermediate layer comprising an electrically insulant cloth which isimpregnated with a sealant that substantially prevents the passage ofliquid through the cloth.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, and with reference to the following drawings and examples.

FIG. 1 shows an exploded section through a prior art assembly of asub-structure and outer skin,

FIG. 2 shows an exploded section through an assembly of a sub-structureand outer skin utilising an embodiment of the present invention,

FIGS. 3, 4 and 5 show sections through first, second and third examplesrespectively of a cloth in accordance with an embodiment of the presentinvention.

DETAILED DISCUSSION OF EMBODIMENTS

Referring now to the drawings in which the same features are denoted bycommon reference numerals:

FIG. 1 shows a prior art structure 1 of part of an aircraft comprising asub-structure 2 and an outer skin 3 which is one of several outer skinpanels to be fitted to the sub-structure 2. The outer skin and thesubstructure may be formed from metal or carbon fibre composite, asdesired. The outer skin panels including outer skin 3, and thesub-structure 2 are manufactured and assembled with adherence to stricttolerance limits. However, due to the cumulative dimensional tolerancesof the sub-structure and the outer skin panels resulting from theirindividual manufacture, there will be a step between adjacent outer skinpanels, which results in unwanted aerodynamic effects which can affectthe performance of the aircraft. For aircraft applications it istypically preferable to have a step of less than 0.5 mm between adjacentpanels.

A resin-impregnated glass fibre cloth 4 has been bonded to the undersideof the outer skin 3, to provide an electrically insulant layer. A shimlayer 5 has been applied to the substructure 2, and has been cured andmachined to tolerance as described earlier in this specification (seepages 1 and 2), to attempt to lessen the step between adjacent outerskin panels. A layer 6 of a sealant such as PRC (Polysulphide RubberCompound) has been applied to the shim layer 5 of the substructureand/or the glass fibre cloth bonded to the outer skin to prevent theingress of water or the egress of liquids such as aircraft fuel or oil.The sealant is of a viscous liquid form for easy application, but has atendency to be squeezed out of the joint during assembly.

FIG. 2 shows a structure 7 of part of an aircraft in accordance with thepresent invention, comprising a substructure 8 and an outer skin 9 whichis one of several outer skin panels to be fitted to the sub-structure 8.The outer skin and the sub-structure may be formed from metal or carbonfibre composite, as desired. The outer skin panels including outer skin9, and the sub-structure 8 are manufactured and assembled with adherenceto strict tolerance limits. If desired, a shim layer 11 (shown as adotted line) may be applied to the substructure and cured and machinedprior to assembly. However, in most cases, it will not be necessary ifthe present invention is employed.

Prior to assembly of the structure, an electrically insulant woven fibrewhich was impregnated with a sealant such as PRC that substantiallyprevents the passage of liquid through the resultant cloth 10 isdisposed on either the substructure or the underside of the outer skin,so that on assembly the cloth 10 will be located between thesubstructure and the outer skin, thereby forming part of the joint. Thecloth in this example is glass fibre cloth which has good insulantproperties. The cloth was made by impregnating woven glass fibre with asealant, instead of with the conventional resin. Resin allows water andother liquids to seep through the material, is so is not suitable foruse as a sealant. Impregnating the woven glass fibre with a sealant suchas PRC instead of resin allows the resultant cloth to have bothimpermeability and insulant characteristics.

FIG. 3 shows an electrically insulant, impermeable glass fibre cloth 12,suitable for use in the manufacture of assemblies in accordance with thepresent invention. The cloth 12 may comprise one or more layers of wovenglass fibre impregnated with sealant.

FIG. 4 shows an electrically insulant, impermeable glass fibre cloth 13which comprises a layer 14 of woven glass fibre impregnated withsealant, and further comprises a layer 15 of partially cured sealant.The layer 14 is formed first, by impregnating the woven glass fibre withsealant and then curing it. The layer 15 is then formed by addingsealant to one face of the resultant layer 14 and the cloth is thenpartially cured. The partially cured layer 15 may be of the same ordifferent thickness to the layer 14 and is readily compressible. Thethickness of layers 14 and 15 may vary along their lengths and widths,dependent upon the requirements of the assembled structure. Thethickness of layer 14 may be approximately 0.25 mm and the thickness ofthe partially cured layer 15 may be approximately 1.5 mm for example.

FIG. 5 shows an electrically insulant, impermeable glass fibre cloth 16similar to that of FIG. 4. The cloth 16 comprises a layer 14 of wovenglass fibre impregnated with sealant, and also comprises a layer 15 ofpartially cured sealant. The cloth 16 further comprises a pre-curedlayer of sealant 17. The layer 17 is impermeable, and enhances theimpermeability of the cloth, which is desirable for use in areas wherethe ingress of water would be very serious. The relative thicknesses ofthe layers 14, 15 and 17 differ, and the thicknesses of each of thelayers may differ along their length.

The cloth is made by first forming the layer 14, by impregnating thewoven glass fibre with sealant and then curing it. The layer 17 is thenformed by adding sealant to one face of the layer 14, and fully curingthe resultant cloth. The layer 15 is formed lastly, by adding sealant tothe other face of the layer 14 and partially curing the cloth to arriveat the finished cloth 16.

It can be seen that adjacent outer skin panels in an assembly may needto be adjusted to fit flush together without a step between them whenthey are assembled to a substructure. The prior art uses machinablesubstructures or shims to achieve this. The present invention allowsadjustment of the outer skin panels simply by utilising differentpressures on each of the outer skin panels during assembly, so that eachcompresses the partially cured sealant by a different amount, allowingthe outer skin panels to be assembled within very tight tolerances.

The partially cured sealant layer will have adhesive properties, and sothe cloth may be easily stuck to the substructure without falling offduring assembly, prior to curing of the structure. Advantageously thecloth may be pre-cut into gaskets designed to fit a particular part ofan assembly, and the gaskets may be affixed to the substructure prior toassembly and curing of the structure.

1. A layer of electrically insulant woven cloth impregnated with anon-epoxy curable sealant that substantially prevents the passage of atleast one of water, aircraft fuel and oil through the cloth, theimpregnated electrically insulant cloth being substantiallycompressible, between two outer faces of the layer, wherein said clothcomprises at least one layer of woven cloth impregnated with saidsealant and further comprises a layer comprising a partially curedsealant.
 2. An electrically insulant woven cloth as claimed in claim 1wherein said layer comprising partially cured sealant does not havesubstantially constant thickness.
 3. An electrically insulant wovencloth as claimed in claim 1 wherein said at least one layer of wovencloth is impregnated with a precured sealant.
 4. An electricallyinsulant woven cloth as claimed in claim 1 wherein said partially curedsealant has adhesive properties, and said layer of partially curedsealant forms an outer layer of the cloth.
 5. A method of making a layerof compressible, electrically insulant and impermeable woven clothcomprising at least the steps of: substantially impregnating a layer ofelectrically insulant woven cloth with a non-epoxy sealant thatsubstantially prevents the passage of at least one of water, aircraftfuel and oil through the cloth, curing the impregnated electricallyinsulant cloth, adding a layer of sealant to the cured impregnatedelectrically insulant cloth, and partially curing said layer of sealant,the cloth being substantially compressible, between two outer faces ofthe layer, to a desired thickness.
 6. A method of making an electricallyinsulant and impermeable woven cloth as claimed in claim 5 comprisingthe further steps of adding a first layer of sealant to a first side ofthe cured impregnated electrically insulant woven cloth, and fullycuring the first layer of sealant, then adding a second layer of sealantto a second side of said cloth and partially curing the second layer ofsealant.